Effect of Deep Groundwater on Karstification of the Lower Ordovician Carbonate Rocks in Tazhong Area

The Lower Ordovician carbonate rocks in the Tazhong area is one of the most important oil and gas reservoirs in the Tarim Basin of China. The heterogeneity of the rocks is very strong. There are mainly three types of karstification: syn-sedimentary karstification, unconformity karstification, and burial karstification in this area. Karstification has an important role in controlling the development and distribution of karst reservoir. The precipitation trend of the buried carbonate calcite or dolomite is affected by the deep groundwater and may control the development and distribution of the karst reservoir. Using geochemical modeling theory of water-rock interaction and combined with the depth and the geothermal gradient, calculation of geochemical thermodynamic parameters of interaction of deep groundwater and calcite or dolomite is carried out. The results suggest that modern burial environment for calcite is in a dissolving tend, and for dolomite in precipitation tend. Strong dissolution trend area of calcite occurs mainly along the fault zone, i.e., along the Tazhong 1 fault and the intersection part of the Tazhong 1 fault and the Tazhong 2 fault.